LDE conducted an independent geotechnical assessment of two 5m high earth dams in a recently formed subdivision.  The principal purpose of the assessment was to review the stability of the dams.

The dams (upper and lower) were constructed in the late 1990s to control stormwater as part of the formation of the Northland subdivision.

An essential part of this project involved the review of historical data such as the original design, survey data, and discussions with personnel involved with the dam's original construction.  In addition to this information, comprehensive testing, analyses, and modelling helped to understand the nature of the site and form the appropriate recommendations and remediation design.

The Upper Dam exceeded the minimum safety criteria under the existing extreme seismic conditions and considered no stability improvements required for the dam.

The lower dam was considered marginal under the seismic conditions modelled and calculated to have inadequate safety factors.  Therefore, subsurface drainage comprising deep filtered buttress drain with finger drains tapping zones of seepage was recommended for this dam.

Location: Whatuwhiwhi, Northland
Client: CPG